This invention relates to electric oscillators. In known oscillators, following the instant of oscillator switch-on, the build-up to maximum oscillatory amplitude takes several cycles of oscillation. Similarly, following switch-off, the amplitude of the oscillatory voltage decays slowly over several cycles of oscillation. With such oscillators, there are many applications in which the oscillator output cannot be used during the aforesaid build-up and decay periods.
In the case of oscillators employed in vehicle detection apparatus, it is particularly desirable to reduce or eliminate the duration time of build-up and decay. This is particularly so in vehicle detection equipment where several channels are repeatedly scanned in sequence on a time-sharing basis and an oscillator is used for each channel for the detection of vehicles. As the vehicles to be detected may be travelling at high speed, the time available for each scan period is limited so that oscillation build-up and decay times further limit the performance of the equipment.
An object of the present invention is to reduce the oscillation build-up time of an oscillator.
In accordance with one aspect of the present invention, there is provided an oscillator comprising;
a supply source of substantially constant current incorporating a control element by which the current magnitude may be controlled, a tuned resonant circuit and switching means for changing the operating mode of the oscillator from a standby mode to an oscillatory mode and vice versa by switching the control element from one operating condition to another so that, during the stand-by mode, a charging current is supplied from the current source to the inductance of the resonant circuit. Upon a transition from the standgy mode to the oscillatory mode the charging current supply to the inductance is interrupted. A control loop is provided between the resonant circuit and the control element for supplying a control voltage to the control element which is dependent upon the voltage across the resonant circuit whereby, during the oscillatory mode, a pulse of current is supplied from the current source to the resonant circuit on alternate half-cycles of the oscillatory voltage developed across the resonant circuit.
In one form of the invention, the said control loop includes a keying signal generator connected to key the said control element in synchronism with alternate half-cycles of the oscillatory voltage developed across the said resonant circuit.
It is desirable to stabilize the amplitude of the oscillator output. In accordance with another aspect of the invention, an amplitude stabilization, sensitive to the peak-to-peak amplitude of the oscillatory voltage across the said tuned resonant circuit, is provided to control the amplitude of the keying pulses supplied by the said keying signal generator to key the said control element The amplitude of the keying pulses are controlled in a manner such that the keying pulse amplitude varies inversely with the peak-to-peak amplitude variation of the oscillatory voltage.
In a practical embodiment in accordance with this aspect of the invention, the keying signal generator includes a differential amplifier. The differential has a first input connected to a reference voltage and a second input connected to monitor the oscillatory voltage across the said resonant circuit. The output of the differential amplifier is connected to supply keying pulses to the said control element with the differential amplifier operating to supply keying pulses having a magnitude dependent upon the difference between the peak-to-peak voltage of the oscillatory voltage across the resonant circuit and the reference voltage.
It is also desirable, when the oscillatory is in the oscillatory mode, to reduce the period of time for oscillation to cease following selection of the standby mode. This may be achieved in accordance with a further aspect of the present invention by the addition to the said switching means of a controlled delay means having a first input connected to sense the selected mode of operation and a second input connected to monitor the voltage present across the resonant circuit. The controlled delay means operator when the standby mode is selected, and the oscillator is in the oscillatory mode, to delay transition to the standby mode and the supply of charging current to the inductance of the resonant circuit, until the flow of current in the said inductance due to oscillatory current flow in the resonant circuit is substantially at a maximum in the direction opposite to that of the charging current.